In a ranking that compares the change in comparative income levels from 2007 to 2012, Milwaukee came in at No. 6 of the 50 biggest cities, according to the study from the Brookings Institution, a public policy think tank in Washington, D.C.

Reasons for the deepening income gulf vary from city to city, Brookings said. In Milwaukee's case, as well as similar industrial Midwestern cities such as Cleveland and Indianapolis, "poverty deepened as local manufacturing industries declined during the recession," the report found.

Separately, the Brookings study offers a snapshot of the top 50 cities based on a ratio of income inequality for 2012. In that ranking, Milwaukee falls in the middle at No. 28.

Economic divergence in urban areas — which dates back to the 1859 novel, "A Tale of Two Cities," and earlier — is hardly a new issue. But it is a topic of fierce debate.

The notion of inequality has become central to the recent national political discussion,most often framed in terms of society's "1% vs. 99%" debate.

"America risks becoming a 'Downton Abbey' economy," according to a commentary this week from Lawrence Summers, former U.S. Treasury secretary and former president of Harvard University.

President Barack Obama drew attention to economic mobility and income inequality in a speech in December, when he called upward mobility "the defining challenge of our time." He again drew attention to the issue this month with his support of an increase in the minimum wage.

In its report, Brookings defines income inequality as a "95/20 ratio," which compares a household that makes more than 20% of income earners in a city — the 20th percentile — vs. a household that makes more than 95%, the 95th percentile.

Thursday's report measures the incomes of only residents inside the city limits, which excludes all suburbs and greater metropolitan areas. The exclusion of suburbs skews data in several ways, the report said: "Like San Francisco, Seattle may have less poverty not only because people there earn more, but also because the region's poor increasingly live in suburbia."

Thursday's report is titled "All Cities Are Not Created Unequal."

"Big cities remain more unequal places by income than the rest of the country," it concluded. "However, some cities are much more unequal than others."

Wide inequality is largely a phenomenon of the East and West coasts. The cities with the widest income gaps in 2012 were Atlanta; San Francisco; Miami; Boston; Washington, D.C., and New York.

Most cities of the Midwest fell somewhere in the middle of the rankings. Chicago, which came in at No. 8, was the Midwestern exception.

San Francisco is singled out in the report. Not only is the Bay City No. 2 in the 2012 income inequality ratio, but it ranks No. 1 among cities where incomes diverged most quickly. "San Francisco's ratio is high because its wealthy households have very high incomes, considerably higher than in any other major city," it said.

About John Schmid

John Schmid covers economics, business and other topics for the Journal Sentinel.